NPC movement and reactions are governed by a set of rules known as Artificial Intelligence or AI. A NPC will walk, run, shoot, or find cover based on a multitude of external stimuli. Each NPC has different AI associated with it.
You could place a generic NPC in your map to add some more immersion, yet without scripts, they won't do too much. As an example, combine soldiers and headcrabs will attack if they see you, but citizens will look at you and acknowledge your presence. These are just default behaviors, but thankfully, we don't have to fully rely on plain NPC AI. We can create scripts to get the NPCs to walk, talk more, and interact with the world. Making NPCs walk is quite easy. The path_corners entity can be used as navigation points for NPCs. There are two main ways to get an NPC to follow a path: with spawn activity and with an aiscripted_schedule.